1. Field of the Invention
The present invention relates to a capacitive sensing device and method, and more particularly, to a capacitive sensing device and method for sensing a change in capacitance which is caused by contact with a human body.
2. Description of the Related Art
In recent years, touch sensors for sensing contact with a human body based on a change in capacitance of an electrode are becoming widely popular. For example, the touch sensors are used for, for example, a mobile telephone, a digital music player, and a portable electronic device. The devices need to be driven with a battery, and hence a reduction in power consumption is desired. However, when the change in capacitance is to be sensed, it is normally necessary to charge and discharge a capacitor. Therefore, the measurement takes a certain time, thereby consuming power. Thus, it is necessary for the devices to minimize a time required to sense the change in capacitance in the touch sensors.
A method of sensing a capacitance using a switched capacitor is described in “Cypress's CapSense Approximation Algorithm” (Cypress Semiconductor Cooperation). FIG. 14 illustrates a sensing circuit disclosed in “Cypress's CapSense Approximation Algorithm” (Cypress Semiconductor Cooperation). In FIG. 14, capacitors CMod and CInternal are provided in the sensing circuit and a capacitor (capacitance) CX is for a measurement object. When a human body (finger) is located over the sensor, a value of the capacitor CX increases. The sensing circuit operates as follows. First, switches φ1 and φ2 are alternately turned on/off. That is, the capacitor CX operates as a switched capacitor. In this case, a resistance value of the capacitor CX is equal to R (=1/(Fs·CX)) (Fs: switching frequency of switches φ1 and φ2). When the human body (finger) is located over the sensor, the value of the capacitor CX increases, and hence the resistance value R reduces. Therefore, a charging time required to reach Vref (predetermined voltage value) in the case where the finger is not located over the sensor is different from a charging time required to reach Vref in the case where the finger is located over the sensor. A time required to charge the capacitor up to Vref is measured using a timer, whereby whether or not the finger is located over the sensor may be sensed.
A mobile telephone, a digital music player, and a portable electronic device for which the touch sensors are widely employed require a large number of switches (bottoms) as user interfaces, and hence the touch sensors are provided for the switches.
When the sensor disclosed in “Cypress's CapSense Approximation Algorithm” (Cypress Semiconductor Cooperation) is used for the large number of switch devices, measurement of capacitances corresponding to the number of switches is required, that is, n-time measurement is required in a case of n-channel switches. This means that discharging and charging of each of the capacitors CX, CMod, and CInternal are performed n times and is a factor inhibiting the reduction in device power consumption.